5.3.4.3.1 General

To support radio resource management in NG-RAN the AMF provides the parameter 'Index to RAT/Frequency Selection Priority' (RFSP Index) to NG-RAN across N2. The RFSP Index is mapped by the RAN to locally defined configuration in order to apply specific RRM strategies, taking into account any available information in RAN. The RFSP Index is UE specific and applies to all the Radio Bearers. Examples of how this parameter may be used by the RAN:

• to derive UE specific cell reselection priorities to control idle mode camping.
• to decide on redirecting active mode UEs to different frequency layers or RATs (e.g. see clause 5.3.4.3.2).

The HPLMN may set the RFSP Index taking into account the Subscribed S-NSSAIs. The AMF receives the subscribed RFSP Index from the UDM (e.g. during the Registration procedure). For non-roaming subscribers, the AMF chooses the RFSP Index in use according to one of the following procedures, depending on operator's configuration:

• the RFSP Index in use is identical to the subscribed RFSP Index, or
• the AMF chooses the RFSP Index in use based on the subscribed RFSP Index, the locally configured operator's policies, the Allowed NSSAI and any Partially Allowed NSSAI, S-NSSAI(s) rejected partially in the RA, Rejected S-NSSAI(s) for the RA, Pending NSSAI and the UE related context information available at the AMF, including UE's usage setting, if received during Registration procedures (see clause TS 23.502 [3]).

NOTE 1: One example of how the AMF can use the "UE's usage setting," is to select an RFSP value that enforces idle mode camping on E-UTRA for a UE acting in a "Voice centric" way, in the case voice over NR is not supported in the specific Registration Area and it contains NR cells.

The AMF may report to the PCF the subscribed RFSP Index received from the UDM for further evaluation as described in clause 6.1.2.1 of TS 23.503 [45]. When receiving the authorized RFSP Index from the PCF, the AMF shall apply the authorized RFSP Index instead of the subscribed RFSP Index for choosing the RFSP index in use (as described above).

For roaming subscribers, the AMF may choose the RFSP Index in use based on the visited network policy, but can take input from the HPLMN into account (e.g. an RFSP Index value pre-configured per HPLMN, or a single RFSP Index value to be used for all roamers independent of the HPLMN). If the AMF receives authorized RFSP Index value from the V-PCF, the AMF chooses the RFSP index in use based on the authorized RFSP Index value.

NOTE 2: The PCF can provide validity time together with the authorized RFSP Index indicating a change in priority from 5G access to E-UTRAN access as specified in clause 5.17.2.2.

The RFSP Index in use is also forwarded from source to target NG-RAN node when Xn or N2 is used for intra-NG-RAN handover.

The AMF stores the subscribed RFSP Index value received and the RFSP Index value in use. During the Registration procedure, the AMF may update the RFSP Index value in use (e.g. the AMF may need to update the RFSP Index value in use if the UE related context information in the AMF has changed). When the RFSP Index value in use is changed, the AMF immediately provides the updated RFSP Index value in use to NG-RAN node by modifying an existing UE context or by establishing a new UE context in RAN or by being configured to include the updated RFSP Index value in use in the NGAP DOWNLINK NAS TRANSPORT message if the user plane establishment is not needed. During inter-AMF mobility procedures, the source AMF forwards both RFSP Index values to the target AMF. The target AMF may replace the received RFSP Index value in use with a new RFSP Index value in use that is based on the operator's policies and the UE related context information available at the target AMF.

In order to enable UE idle mode mobility control and priority-based reselection mechanism considering availability of Network Slices at the network and the Network Slices allowed for a UE, an RFSP is derived as described in clause 5.3.4.3, considering also the Allowed NSSAI and any Partially Allowed NSSAI for the UE.

A UE supporting NSAG (see clause 5.15.14) may be configured, for some of the S-NSSAIs in the configured NSSAI, with NSAGs it can use as described in TS 38.300 [27], TS 38.304 [50], TS 38.331 [28], TS 38.321 [143], TS 24.501 [47] and as described in clause 5.3.4.3.4.